H. Frazer (Journal of Polymer Science (A2) pp. 1137-1160 (1964)) has described the synthesis of copolyoxadiazoles by reacting diacid chloride with a dihydrazide derivative of another diacid according to the scheme (1), in sulphuric acid: ##STR3## a polyhydrazide is thus obtained, which is isolated and then heat-treated in order to obtain the cyclized oxadiazole form ##STR4##
The structure is alternating, with a unit B following a unit A.
Ueda and Sugita (Journal of Polymer Science A26, 159 (1988)) describe the synthesis of polyoxadiazoles in a single step from diacids and hydrazine sulphate in methanesulphonic acid solvent in the presence of phosphoric anhydride.
The polyoxadiazoles obtained from aromatic diacids have a high thermal stability and are materials possessing a high rigidity. However, these materials cannot in general be melted because their melting point is above 400.degree. C.
The introduction of aliphatic segments, by the use of a mixture of aromatic diacids and aliphatic diacids, leads to a reduction in the melting point of the copolyoxadiazoles. Korshak (Vysokomol Soed. B 10(8) 568 (1988) and Polymer Science of USSR 8, 883 (1966)) has prepared copolyoxadiazoles of alternating type, that is to say that the aliphatic units and the aromatic units are alternately distributed.
The melting points are as follows:
______________________________________ COPOLYMERS MELTING POINT in .degree.C. ______________________________________ 6/I 180 6/T 240 10/I 100 10/T 220 ______________________________________
6 and 10 denote the number of carbons in the aliphatic unit, (I) isophthalic acid and (T) terephthalic acid, the aliphatic and aromatic units are separated by the oxadiazole units ##STR5##
Only one glass transition temperature (Tg) and only one melting point are found for each copolymer.
The Korshak synthesis is effected either by reacting aliphatic dicarboxylic acid dichloroanhydrides with aromatic dicarboxylic acid dihydrazides or by reacting aromatic dicarboxylic acid dichloroanhydrides with aliphatic dicarboxylic acid dihydrazides.
These polyoxadiazoles and copolyoxadiazoles containing alternating aliphatic and aromatic units are either crystalline or amorphous. They do not posses the properties of elastomer materials.